How a wire actually moves a frame from one machine to the next: Ethernet framing, MAC addresses, switches, ARP, VLANs, MTU, and the loops you have to design around.
6 topics
The link layer is where addressing stops being an abstraction and a frame physically leaves a port. Layer 3 decides which network a packet is headed for; layer 2 is what carries it across the single wire or switch fabric in front of it, one hop at a time. Every packet you send is wrapped in a frame addressed to a MAC on the local segment, and that frame is rebuilt from scratch at every hop along the way.
This chapter covers what lives on that wire. Ethernet framing and the 48-bit MAC address; the switch that learns which host sits behind which port; ARP, the unauthenticated lookup that ties an IP to a MAC; VLANs that slice one switch into isolated broadcast domains; the 1500-byte MTU and the fragmentation pain that comes from exceeding it; and spanning tree, the protocol that keeps a redundant layer-2 topology from melting itself into a broadcast storm. Get these right and most "the app is up but unreachable" mysteries resolve themselves.